(1) Field of the Invention
This invention relates to an injection blow molding method for producing plastic flat bottles and the like. More particularly, the invention relates to an improved injection blow molding method in which injection molded cylindrical parisons are blown with air to produce flat bottles which are commonly used as the receptacles for toiletries, detergents and oils.
(2) Description of the Prior Art
It has been easily and commonly practised to produce cross-sectionally circular bottles by inflating injection molded cylindrical parisons having bottoms (hereinafter referred to as simply "parisons") with blowing air into them.
Further, when the cross-sectionally circular bottles are molded from the cylindrical parisons, the wall thicknesses of the bottles become even because the ratio of expansion is constant if the thickness and temperature distribution of the parison is even and there occurs no difference in the expansion of each portion of the parison during the blowing.
Meanwhile, when the bottle having a cross-section in the shape of an ellipse or a rounded rectangle (hereinafter referred to as "flat bottle") is produced from the cylindrical parison, the distribution of wall thickness becomes irregular and the uneven thickness is caused to occur because the ratio of expansion is uneven.
When the bottle of thick wall is molded, this unevenness in wall thickness is not so notable and there occurs no serious problem in view of the wall thickness. However, when a bottle of 0.2 to 0.5 mm in wall thickness is produced in which the parison is expanded in both the directions of its axis and circumference, the unevenness in wall thickness becomes serious. In an extreme case, both side end portions become very thin like a film so that the molded product losses its utility as a bottle.
Since the cause of such uneven thickness is attributable to the deformation of the cross-section of the parison from a circle into an ellipse, as the measure to avoid the uneven thickness, it will be considered that the problem may be solved by forming the parison in accordance with the shape of the final product. In the injection molding, however, the neck portion is integrally formed, which is different from the extrusion molding, therefore, it is impossible to form such differently shaped parison due to the structure of the mold.